This invention relates to semi-permeable membranes derived from polycarbonates wherein the polycarbonates are derived in a significant portion from tetrahalobisphenols. The invention further relates to the use of these membranes to separate certain gases, for example, oxygen from nitrogen, and carbon dioxide from methane.
In various industries, it is necessary or highly desirable to separate one component from another in a gaseous stream. Processes used to perform such separations include pressure swing adsorption and membrane separations. In a membrane separation, a gaseous stream containing the components to be separated is contacted with a membrane, wherein the membrane separates two regions in a manner such that only those materials which permeate through the membrane can communicate from one region to the other. Such membranes are semi-permeable, in that one component of the gaseous mixture selectively permeates through the membrane at a rate much higher than one or more of the components in the gaseous stream. The gaseous mixture is contacted with the membrane in a manner such that the selectively permeable species is preferentially transported through the membrane to the other region. It is to be noted that the component from which the selectively permeable species is to be separated may in fact permeate through the membrane at a much slower rate than the selectively permeable species. It is this difference in rates of permeation which is used to separate the gaseous species or reduce the concentration of the less selectively permeated species in the region to which the permeating gases permeate.
In such separations, the relative rate of permeation, that is, the difference in rate of permeation between the selectively permeating gas and the non-selectively permeating gas, is a major factor in the separation achieved. The higher the ratio of permeation of the selectively permeable gas over the non-selectively permeable gas, the better the membrane will perform. Therefore, it is desirable to have as high a ratio as possible.
Presently, membranes derived from acetate esters, for example cellulose triacetate, and olefins, for example polyethylene, polypropylene, poly-4-methylpentene-1, are used for gas separations. Among such separations are the separation of oxygen from nitrogen, and carbon dioxide from methane.
Some of the materials used in membranes suffer from certain disadvantages. One such problem is the inability to perform under extreme conditions, such as high temperatures and pressures. As a result, certain separations are limited by the membrane as to the temperatures and pressures which may be used.
What are needed are membranes with a high relative rate of permeation through the membrane of the selectively permeating species over the non-selectively permeating species. Further, what is needed is a membrane which has enhanced mechanical strength so as to withstand more extreme temperatures and pressures.